This invention relates to a row crop guidance system for maintaining the alignment of a pull type forage harvester with the crop rows being harvested. More particularly, the invention relates to an electrohydraulic control circuit for such a guidance system.
One type of commercially available pull type forage harvester which is used to harvest a variety of row crops such as corn, sorghum, sugar cane and the like includes a base unit and an attached header. The base unit/header combination is towed through a field with a tractor. In the harvesting mode, the harvester is positioned outboard of the tractor by operation of a hydraulic cylinder connected to the tongue of the harvester so that the towing tractor does not have to ride over the crop prior to harvesting. When harvesting, the operator must look behind himself frequently to maintain the alignment of the harvester with the crop rows being harvested. After initially aligning the tractor and harvester properly relative to the crop rows, alignment is usually maintained by steering the tractor. However, the requirement for the operator to look over his shoulder frequently at the harvester leads to operator fatigue. When not in use the harvester is also positionable behind the tractor for transport.
To avoid operator fatigue and to simplify the harvesting operation, it has been proposed to provide a guidance system for automatically maintaining the alignment of a pull type harvester with crop rows being harvested. One proposal is disclosed in U.S. Pat. No. 4,295,323. One problem in the design of such systems is that the system must provide reliable operation under a variety of crop and field conditions including good standing and down crop conditions, clean or weed and grass infested fields and smooth to very rocky fields. In addition, stalk strength varies with the type of crop, and crops of the same type have varying stalk strengths due to exposure to different growing conditions including factors such as moisture level, amount of fertilizer used, cultivation practices and thickness of plant.